Process for isomerizing alkyl benzenes



3,172,918 PROCESS FOR ISOMERIZING ALKYL BENZENES Peter Fotis, Jr., Highland, and Robert M. Aim, rown Point, Ind., assignors to Standard Oil Company, Chicago, 111., a corporation of Indiana No Drawing. Filed Jan. 31, 1961, Ser. No. 86,011 Claims. (Cl. 260-668) This invention relates to a method of isomerization and particularly relates to a method of isomerizing benzenes having a plurality of substituted alkyl radicals using a heteropoly acid catalyst.

In petroleum refining and particularly in petrochemicals manufacture, it is frequently desirable to convert a compound or mixtures of compounds to isomers thereof. For a specific example, reference is made to the manufacture of para-xylene. Para-xylene is extensively used as a feed stock for the manufacture of terephthalic acid, which in turn is used in the manufacture of synthetic fibers. Para-xylene may be separated by crystallization from a mixture of aromatic hydrocarbons having 8 carbon atoms per molecule. As a result of the separation of para-xylene from such a mixture, there remains a mother liquor comprising a mixture of such isomers which is lean in para-xylene, but which comprises about 3 to 4 volumes per volume of separated para-xylene. It would, of course, be advantageous to be able to convert the isomers in the mother liquor to additional para-xylene which could then be separated as indicated above. Such a conversion is particularly desirable in View of the fact that the C aromatic isomers other than para-xylene are of little value, in the current state of technology, in the manufacture of synthetic fibers, and are actually used for purposes wherein their economic value is much less than is the value of para-xylene.

We have now discovered a process wherein a feed comprising a major proportion of a benzene substituted with a plurality of hydrocarbon radicals may be isomerized to yield an isomerization product characterized by having a composition which approaches the equilibrium composition of the isomers of the feed benzene. Such isomers have the same alkyl radicals as the feed benzene, but the radicals are attached to the benzene ring at various positions. The catalyst used in our isomerization process is a heteropoly acid. The isomerization conditions used comprise liquid phase operation, temperatures within the range of about 200 to 400 C., and liquid hourly space velocities in the range of about 0.1 to 2.

Suitable feeds for our process comprise predominantly hydrocarbon materials having, preferably, a major proportion of one or more benzenes having a plurality of alkyl radicals. The radicals of the feed benzene may be in any position, such as 1,2, or 1,3, or, in the case of benzenes having 3 substituents, 1,3,5. The alkyl radicals attached to the feed benzene may or may not have the same number of carbon atoms per radical. Also, even when the radicals do have the same number of car bon atoms per radical, the structural configuration may be different, for example, propyl and isopropyl radicals.

After undergoing isomerization in our process, the isomerization product will have a composition approaching the equilibrium composition of the isomers of the feed benzene, such isomers having the same alkyl radicals as the feed benzene, but having the radicals in differing positions around the benzene ring. By way of illustration, 1,2, di-methyl benzene (ortho-xylene) when used as a feed to our process will be isomerized to a mixture of itself and 1,3 di-methyl benzene (meta-xylene) and 1,4 di-methyl benzene (para-xylene). The equilibrium composition at the isomerization temperatures referred to herein of such isomers comprises about 23 percent paraxylene, about 14 percent ortho-xylene, and the remainder 3,172,918 Patented Mar. 9, 1955 meta-xylene, Data on the equilibria compositions of various aromatics, including di-methyl benzene, is published in Journal of Research of the National Bureau of Standards 37, 95 (1946) in an article by W. J. Taylor et al. Additional examples of plurally alkyl substituted benzenes which may be isomerized in our process are the tri-methyl benzenes, hemimellitene, pseudocumene, and mesitylene, the methyl ethyl benzenes, the methyl propyl benzenes, the methyl isopropyl benzenes (ortho-, meta-, and para-cymene), the di-ethyl benzenes, the di-methyl ethyl benzenes, and the tetramethyl benzenes (prehnitene, iso-durene, and durene). The benzenes for which the process is applicable are not limited to the foregoing illustrative examples, but include benzenes having alkyl radicals containing a larger number of carbon atoms, such as the butyl, pentyl, hexyl, heptyl, and octyl radicals. When the feed to the process comprises benzenes having different alkyl substituents, for instance, a di-methyl benzene and a methyl propyl benzene, the reaction product will comprise the di-methyl isomers and the methyl isopropyl isomers in equilibrium with each other.

Although it is preferable to have a feed which comprises only one or more of the desired isomers, the presence in the feed of other hydrocarbon constituents is not harmful, because these other constituents will merely pass through the reactor substantially unchanged. It is, however, desirable to avoid to the extent possible basic constituents capable of neutralizing the heteropoly acid catalyst. For this reason it is desirable to exclude ammonia, other amines, hydroxides, and other bases from the feed. The presence in the feed of small amounts of acids or water is not objectionable.

The catalyst used in our process is a heteropoly acid. Poly acids are complex molecules in which a plurality of simple acid molecules apparently have been condensed to give more than a single mol of acid anhydride. When only a single type of acid anhydride is involved, the acid is designated an isopoly acid; if more than a single type of acid anhydride is present the acid is termed a heteropoly acid. The preparation and structure of heteropoly acids are described in numerous standard chemistry texts. Various heteropoly acids are available as items of commerce.

In heteropoly acids, one of the acid forming elements is designated as the central acid element, while the remaining acid elements are designated outer acid forming elements. The latter are bound to the former in varying ratios, generally multiples of 3, such as 6:1 or 12:1. The central acid forming element may be any element which is at least trivalent, and is capable of forming an oxygen containing compound which has acidic properties. Suitable central acid forming elements are aluminum, chromium, cobalt, platinum, antimony, and, advantageously, phosphorus, borium, silica, and arsenic. Suitable outer acid forming elements are tungsten, vanadium, and molybdenum. Illustrative examples of preferred heteropoly acids which may be used as catalysts in the isomerization process are phosphot-ungstic acid, phosphovanadic acid, and phosphomolybdic acid, and the equivalent acids wherein silica or boron is substituted for phosphorus. A particularly advantageous acid is phosphotungstic acid.

The heteropoly acid may be used alone or supported on an inert carrier, such as alumina, silica, fullers earth, or kieselguhr. Carriers having relatively high surface areas and large pore diameters are preferred. that the carrier, if used, be free of substances which would tend to neutralize the heteropoly acid. A suitable means to assure that the carrier is free of alkaline-reacting substances is to treat'it with a dilute acid, such as aqueous pellets, granules, or a fiuidizable powder. The amount It is desirable of heteropoly acid on the carrier may vary widely from about to about 50 weight percent of the carrier, preferably to about 35 percent, and advantageously to percent. The heteropoly may be added to the carrier by dissolving a heteropoly acid, or mixtures of such acids, in a solvent, such as water, an alcohol, an ether or a ketone, and then slurrying the resulting solution with the carrier. Thereafter, the carrier is separated from the solution and dried at moderate temperatures to evaporate uncombined water without driving off combined water. The drying temperature may be in the range of about 100 C. to 400 C. at atmospheric pres sure, preferably at about 110 to 200 C. Lower temperatures should be used if the drying is done at less than atmospheric pressure. It is preferable to use relatively lower drying temperatures in order to avoid dehydrating the heteropoly acid to form anyhdrides. The various heteropoly acids start to dehydrate at temperatures above about 500 C. However, if the catalyst has been partially decomposed to anhydrides as a result of exposure to excessive temperature, it can be reconstituted by steaming.

The activity of the catalyst is decreased as a result of the deposition of coke or other carbonaceous material thereon. In the event a catalyst becomes deactivated because of excessive coke deposits, the heteropoly acids may be recovered from the coked catalyst by extraction with a solvent such as water, ethers, alcohols, or ketones, and again used to manufacture new catalyst.

Our isomerization process is conducted in the liquid phase at temperatures in the range of about 200 to 400 C. and at liquid hourly space velocities in the range of about 5 to 2, preferably in the range of about A1 to 1, and, advantageously, about /2. As used herein, liquid hourly space velocities designate liquid volumes of hydrocarbon feed per hour per bulk volume of catalyst. The pressures used may be widely varied, provided, of course, that the pressure is suificient to maintain the feed and the isomerized product in the liquid phase. We have found pressures in the range of about 300 to 1000 p.s.i.g. to be useful. Within the foregoing temperature range of about 200 to 400 C. it is pref erable to operate with temperatures between about 250 to 350 C., desirably between 275 to 320 C. It is to be understood that the temperature used will be below the critical temperature of the feed. Although the hydrocarbon feed and isomerization product should be maintained in the liquid phase, it is to be understood that our process may be conducted in the presence of so-called permanent gases, such as nitrogen, hydrogen, and methane.

Our process may be conducted batchwise or, more preferably, in a continuous flow system. In a continuous flow system, the catalyst may be in a fixed or moving bed. By way of illustration, a plant for conducting our process would comprise a reactor adapted for high pressure operation and in which is disposed a bed of the heteropoly acid catalyst supported on a carrier such as silica gel or alumina. The feed, suitably heated to the desired temperature, may be pumped through the catalyst bed in either an upfiow or a downflow direction. Optional feed pretreatment facilities may include distillation towers, driers, acid treaters, and the like. The effluent from the reactor may be used directly in further processing or cooled and/or distilled to recover particular constituents.

A series of experiments were conducted to demonstrate the operability of the process. A commercially available phosphotungstic acid was purified by dissolving in dilute aqueous HCl. An undissolved residue was separated from the solution by filteration. The filtrate, comprising an aqueous solution or phosphotungstic acid, was extracted withether, in which the phosphotungstic acid is preferentially soluble. The purified phosphotungstic acid was recovered by evaporation of the ether.

In preparing the catalyst, the phosphotungstic acid,

purified as described above, was dissolved in water, silica gel particles having a mesh size of between about 60 to (ASTM Test E-ll), and an average pore diameter of 140 angstroms, was slurried with the aqueous phosphotungstic acid solution, after which the water was evaporated therefrom. The catalyst preparation was completed by drying at about 284 F. and a pressure of 1 mm. of mercury for about 2 to 4 hours. The completed catalyst comprised 20 parts by weight of phosphotungstic acid per 100 parts of silica gel.

A series of batch experiments were conducted with phosphotungstic acid wherein single isomers of the dimethyl benzenes were isomerized. The feeds for these runs were ortho-xylene, meta-xylene, and para-xylene. About ml. of the individual xylene isomers were separately charged to rocker bombs, together with 5 grams of the catalyst above described. The bombs were maintained in the liquid phase at 540 F. and 220 p.s.i.g. pressure for a period of about 4 hours, after which the composition of the hydrocarbon contents was determined by infra-red analysis. The results of the rocker bomb experiments are set forth as examples A, B and C in the following table, and illustrate that the heteropoly acid catalyst is effective for isomerizing each of the di-methyl benzene isomers towards the equilibrium mixture of such isomers.

As used herein, the abbreviations 0X, MX, PX, EB, and T01. desigortho-xylene, meta-xylene, para-xylene, ethyl benzene, and toluene, respectively.

Additional experiments were carried out charging metaxylene to a flowing reactor system. The apparatus used was a heated steel vessel having a diameter of one inch, and equipped for high pressure operation. In these runs, the catalyst loadings in the reactor varied between 15 to 30 milliliters. The temperature, pressure and space velocities were varied, as set forth as Examples D through K of Table 11.

These examples show the effect of temperature, pressure and space velocities, and, in particular, the effects of operating within the ranges of temperature and space velocities disclosed herein. The extent of isomerization which occurs at 200 C. is significantly less than that which occurs at 280320 C. However, as the temperature is increased, the rate of undesirable side reactions, such as cracking and disproportionation, also increases. The rate of such side reactions, relative to the rate of isomerization, is also influenced by space velocity: see Examples G and H. Increasing the space velocity to 2 volumes of feed per hour per volume of catalyst, as in Example K, does not provide time for isomerization to equilibrium conditions, although extensive isomerization does occur.

T able H Composition of Reaction Process Conditions Product, percent Example No.

EB Tol. rcssure, Tempcr- LHSV p.s.i.g. ature, C.

400-500 200 1 400-500 280 3 400-500 300 V Tr. 400-500 320 4 400-500 280 $4 800-1, 000 280 l 800-1, 000 280 2 To illustrate the benefits obtainable from our process, a mixture of Xylenes comprising the mother liquor remaining after the separation by crystallization of most of the para-xylene from a substantially equilibrium mixture of xylenes was charged to the flow-system apparatus referred to above. These runs were conducted using a catalyst comprising 20 parts by weight of phosphotungstic acid supported on 100 parts by weight of silica gel. The composition of the feed and the isomerized products are set forth in Table III.

The process conditions used were, for Examples L and M respectively, 280 and 305 C. and 0.56 and 0.5 liquid hourly space velocities. The runs were carried out in the liquid phase, and at pressures between 300 and 700 p.s.i.g.

Although it has been found that such diverse hydrocarbons as n-hexane, n-heptane, and tetralin do not undergo isomerization in our process, it has been found that methyl cyclopentane may be isomerized to cyclohexane using our process. As an example, 30 milliliters of methyl cyelopentane were reacted in a batch system at 250 C. for 20 hours in the presence of a catalyst comprising 0.8 gram of phosphotungstic acid supported on sufficient silica gel to comprise 4 grams of total catalyst. At the end of the run, the reaction mixture had 87.5 percent methyl 6 cyclopentane and 12.5 percent cyclohexane. The methyl cyclopentane-cyclohexane equilibrium composition at 250 C. is :20 respectively.

Having thus described our invention, what is claimed is:

1. An isomerization process which comprises contacting a feed comprising a major proportion of a benzene substituted with a plurality of alkyl radicals with a catalyst comprising a heteropoly acid under isomerizing conditions, which conditions comprise liquid phase operation, tem peratures within the range of about 200 to 400 C. and liquid hourly space velocities in the range of about to 2, and separating from said catalyst a reaction product characterized by having a composition which approaches the equilibrium composition of the isomers of said benzene, said isomers having the same alkyl radicals as said benzene.

2. The process of claim 1 wherein said feed comprises a di-methyl benzene.

3. The process of claim 2 wherein the pressure is between about 300 to 1,000 p.s.i.g., and the temperature is in the range of about 250 to 320 C.

4. The process of claim 1 wherein said catalyst comprises phosphotungstic acid supported on silica gel.

5. A process for isomerizing di-methyl benzenes which process comprises contacting a feed comprising a major proportion of di-methyl benzenes in the liquid phase at temperatures in the range of about 250 to 320 C., at pressures in the range of about to 1,000 p.s.i.g., and liquid hourly space velocities in the range of about A to l, with a catalyst comprising in the range of about 5 to about 50 weight percent phosphotungstic acid supported on silica gel, and separating from said catalyst a reaction product characterized by having a composition which approaches the equilibrium composition of the di-methyl benzene isomers.

References Cited in the file of this patent UNITED STATES PATENTS 2,547,380 Fleck Apr. 3, 1951 2,705,248 Coats Mar. 29, 1955 2,719,183 Boedeker et a1 Sept. 27, 1955 

1. AN ISOMERIZATION PROCESS WHICH COMPRISES CONTACTING A FEED COMPRISING A MAJOR PROPORTION OF A BENZENE SUBSTITUTED WITH A PLURALITY OF ALKYL RADICALS WITH A CATALYST COMPRISING A HETEROPOLY ACID UNDER ISOMERIZING CONDITIONS, WHICH CONDITIONS COMPRISE LIQUID PHASE OPERATION, TEMPERATURES WITHIN THE RANGE OF ABOUT 200* TO 400*C. AND LIQUID HOURLY SPACE VELOCITIES IN THE RANGE OF ABOUT 1/10 TO 2, AND SEPARATING FROM SAID CATALYST A REACTION PRODUCT CHARACTERIZED BY HAVING A COMPOSITION WHICH APPROACHES THE EQUILIBRIUM COMPOSITION OF THE ISOMERS OF SAID BENZENE, SAID ISOMERS HAVING THE SAME ALKYL RADICALS AS SAID BENZENE. 